The invention relates to a colorimeter for measuring colour of process liquid comprising two light sources and two detectors, both light sources being arranged to emit a beam through a process medium and through windows adjoining the process medium to both detectors, the detectors being arranged to convert the beam arriving from the light sources to electrical signal.
Such colorimeters based on the four-ray principle are currently known in connection with various industrial processes and measurements associated thereto. Even though several features affecting the measurement results disadvantageously can in theory be eliminated by the four-ray principle, the drawback of known solutions has, however, been their laborious use, and also the inaccuracy of the measurement in practice. The measurement inaccuracy can be caused by, for example, a light beam that is not an ideal light beam in practice. Inaccuracy can furthermore be caused by used optics that may include local differences i.e. the windows, through which the light beams pass, can be fouled in various ways in different points, the light beams passing through the different points in the window thus attenuating in different ways etc.